This invention relates to an image sensor of the hybrid type having an insulating substrate which supports a number of photoelectric pickup conversion elements arranged in a row and a thin-film IC for control, such as for signal readout. Such sensors are particularly useful in facsimile transmission systems.
Because of the desire for miniaturization and cost-reduction, contact-type image sensors using semiconductors are growing in popularity. Such sensors typically include an array of photoelectric elements or pickups aligned in a row, for example 8 per millimeter or 1728 elements in a width of 216 mm for use with a typical scratch pad. Generally the array of elements is moved past the manuscript to be read a line at a time while the manuscript is illuminated by an array of light-emitting diodes. A lens array is usually used to focus the light reflected from the manuscript on the array of photoelectric elements.
To realize the operation speed desired, the readout time per scan should be of the order of 4 ms or less. To cope with such circumstances, a sensor using amorphous silicon (a-Si) has been developed.
FIG. 2 shows an example of the conventional sensor using a-Si, in which (a) is a top plan view and (b) a sectional view taken along line B--B of (a). On a transparent insulating substrate 1 there are formed photodiodes, each functioning as a sensor pickup element capable of responding to a light signal entering from the side of the substrate, and each comprising a transparent electrode 2 functioning as an individual electrode, an a-Si layer 3, and a metal electrode 4 functioning as a common electrode. The transparent electrode 2 is made of a transparent conductive film 500--2000 .ANG. thick; the a-Si layer 3 is provided in known fashion and includes a p-type layer of about 100 .ANG. thick, an undoped i-layer of about 0.5 .mu.m thick, and n-type layer of about 500 .ANG. thick in this order from the side of the transparent electrode, and the metal electrode 4 has a thickness of about 1 .mu.m. Each of the transparent electrodes 2, of which there are 8 per 1 mm, is composed of an individual electrode portion 21 of 100 square .mu.m and a narrow lead portion 22. These are patterned by the use of the photolithography from a coating of the transparent conductive film. The separate lead portions 22 of the transparent electrode are connected to metal leads 5; each metal lead 5 is for transferring signals to an input pad 16 of an IC 6 for processing light signals from the sensor elements, and its pattern is formed by the use of the photolithography process after evaporation. Connection between the end of each metal lead 5 and the pad 16 of the IC 6 is realized by a bonded wire 7.
FIG. 3 shows an equivalent circuit of each sensor element for use in explaining the signal readout operation. The photodiode 31 is an a-Si diode made up of the foregoing three, p-i-n, layers to which a reverse bias of 5 V is applied. Photoelectric current yielded here is charged into the lead capacitance represented by the lumped capacitor 33 while the switch 32 is open. The charging time corresponds to about the time of one scan and is of the order of 4 ms. These photodiodes 31, analog switches 32, etc. of which there are 1728 are connected in parallel to a current-voltage converting circuit 34. FIG. 2 illustrates one set only. The charge stored in each lead capacitance 33 is discharged through an inductance component 35 of the wiring by turning on the 1728 switches 32 successively; thus, by discharging the integrated current thus stored each time the switch is closed, one-dimensional picture information is provided corresponding to the illumination intensity of the light incident on the succession of photodiodes 31.
Letting I(t) be the charging current, the amount Q of charge accumulated within a certain time t is represented by the following equation: ##EQU1## If I(t) were constant, Q becomes proportional to I; but, it is not actually constant because the voltage at point 36 of FIG. 3 rises during charging. Accordingly, the voltage across the photodiode 31 also changes. That is, I(t) is not constant and is represented by the following equation: ##EQU2## Namely, I(t) decreases at the rate, (.DELTA.I/.DELTA.V).multidot.Q/C, where (.DELTA.I/.DELTA.V) is the slope of the current-voltage characteristic of the photodiode.
FIG. 4 shows the current-voltage characteristic measured in an actual a-Si diode. Even though it depends on the processing conditions of the a-Si, the normalized (.DELTA.I/.DELTA.V) is estimated at about 3% per 1 V. On the other hand, as will be apparent from the equation (2), if the wiring or lead capacitance C were large enough, the change of I(t) would become small. Increasing the length of the metal lead 5 will increase C. However, if the circuit is designed as shown, the inductance component 35 increases correspondingly, and the charge will not be discharged completely when discharging by closing the switch 32. This can lead to errors and to extra noise; thus, the foregoing measure is not practical.
Conversely, it is found that if the length of the lead 5 is shortened, the rate of change of the charging current becomes excessively large at high illumination intensity and the linearity is lost which makes for poor fidelity of picture reproduction.